1. Field of the Invention
The present invention relates to a system for calculating SCR conversion efficiency for diesel vehicles, and a method thereof. More particularly, the present invention relates to a system for calculating SCR conversion efficiency for diesel vehicles and a method thereof that calculate the SCR conversion efficiency by applying a reference control signal by which a urea dosing module is controlled to NOx conversion values respectively in absorption and release modes of NH3 according to a loading amount of the NH3 determined from a selective catalytic reduction (SCR) efficiency map.
2. Description of Related Art
Diesel vehicles are provided with various post-processing apparatus for removing NOx, CO, THC, soot, particulate matter (PM), and other noxious materials contained in exhaust gas in order to meet Tier2 BIN5 standards or EURO 6 standards.
An SCR uses NH3 that is decomposed from a urea solution as a reducing agent for purifying the NOx. The NH3 has very superior selectivity for the NOx, and a reaction is prompted between the NOx and the NH3 when O2 exists.
The reaction between the NOx and the NH3 is as follows.NOx+NH3→N2+H2O
The dosing module injects the urea solution in order to maintain SCR conversion efficiency higher than a predetermined level, and a target amount of the NH3 generated through evaporation and dissolution of the urea solution is loaded within the SCR.
Generally, the SCR conversion efficiency is increased as an injection amount of the urea solution increases, but the generated NH3 is not dissolved or is exhausted into the air without reacting with the NOx when the urea solution exceeding a predetermined amount is injected. Therefore, the environment is contaminated worse than before.
Particularly, the quality of a vehicle is deteriorated when the excessively generated NH3 is exhausted into the air through an exhaust pipe.
Therefore, it is very important for the SCR conversion efficiency to be more precisely calculated such that the injection amount of the urea solution is controlled.
As shown in FIG. 4, in conventional diesel vehicles, the SCR conversion efficiency value is calculated according to an average value of NOx conversion values in an absorption mode A1 of the NH3 when the urea solution is injected, and in an release mode B1 of the NH3 when the urea solution is not injected from an SCR efficiency map established according to a loading amount of NH3, which represents absorbed amount of the NH3 in the SCR.
For example, since the NOx conversion value X1 in the absorption mode A1 is 90% and the NOx conversion value Y1 in the release mode B1 is 60% in the SCR efficiency map when the temperature of the SCR is 225° C. which is an activation temperature and the loading amount of the NH3 is 1% (1000 mg), the SCR conversion efficiency Z1 may be 75% that is an average value of the NOx conversion value in the absorption and release modes.
Such conventional methods have a vast difference from an actual SCR conversion efficiency if the urea solution is injected or not. Therefore, when the average value is assumed as the SCR conversion efficiency, safety and reliability of actual SCR control cannot be guaranteed and the SCR conversion efficiency cannot be calculated from a characteristic change according to an intermittent control of the dosing module.
In other words, according to a conventional method for calculating the SCR conversion efficiency, a PWM signal of 50% for controlling the dosing module is applied, and therefore there may be a large error from the actual SCR conversion efficiency generated in the SCR.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.